1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a system and method that provides thermal protection for switches.
2. Description of the Related Art
Switching power converters convert power received from a power source, such as a voltage supply, into power suitable for a load. The power received from the voltage supply is referred to as “POWER IN”, and the power provided to the load is referred to as “POWER OUT”. All switching power converters have some inherent power losses due to, for example, non-ideal component characteristics. Such inherent power losses tend to be minimized so as to increase the efficiency of the switching power converters. Inherent power losses are represented herein by “PINH”. In some contexts, the amount of power supplied to the switching power converter can exceed the amount of power provided by the switching power converter to a load, i.e. POWER IN>POWER OUT+PINH. When the POWER IN is greater than the POWER OUT+PINH, the switching power converter passively dissipates the excess energy using passive resistors.
A dimmable lighting system that includes a low power lamp, such as one or more light emitting diodes (LEDs), represents one context when the POWER IN to the switching power converter can be greater than the POWER OUT PINH of the switching power converter. In this exemplary context, the switching power converter receives current through a triode for alternating current (“triac”) based dimmer. Once a triac-based dimmer begins conducting during a cycle of an alternating current (“AC”) supply voltage to prevent the triac from disadvantageously, prematurely disconnecting during mid-cycle of the supply voltage, the switching power converter draws a minimum current referred to as a “hold current”. As long as an input current to the switching power converter is greater than or equal to the hold current, the triac-based dimmer should not prematurely disconnect. For a leading edge dimmer, a premature disconnect occurs when the dimmer begins conducting and stops conducting prior to reaching a zero crossing of the supply voltage. Premature disconnects can cause problems with the lighting system, such as flicker and instability.
Thus, to prevent premature disconnection of the triac-based dimmer, the minimum POWER IN to the switching power converter equals the hold current (“iHOLD”) times an input voltage “VIN” to the switching power converter. Conventional triac-based dimmers were designed to provide power to incandescent light bulbs. For desired dimming levels, an incandescent light bulb generally draws a current at least equal to the hold current for all usable dimming levels. However, other lamps, such as LEDs are more efficient than incandescent light bulbs in terms of power versus light output and, thus, provide equivalent light output while using less power than an incandescent light bulb. Thus, lighting systems with LEDs typically utilize less power and less current than incandescent bulbs. To balance the power when the lighting system draws more POWER IN than the lighting system inherently dissipates and provides as POWER OUT to the lamp, the lighting system utilizes one or more passive resistors to internally dissipate excess power.
FIG. 1 depicts a lighting system 100 that includes a leading edge, phase-cut dimmer 102. FIG. 2 depicts ideal, exemplary voltage graphs 200 associated with the lighting system 100. Referring to FIGS. 1 and 2, the lighting system 100 receives an AC supply voltage VIN from voltage supply 104. The supply voltage VIN, indicated by voltage waveform 202, is, for example, a nominally 60 Hz/110 V line voltage in the United States of America or a nominally 50 Hz/220 V line voltage in Europe. A leading edge dimmer 102 phase cuts leading edges, such as leading edges 204 and 206, of each half cycle of supply voltage VIN. Since each half cycle of supply voltage VIN is 180 degrees of the input supply voltage VIN, the leading edge dimmer 102 phase cuts the supply voltage VIN at an angle greater than 0 degrees and less than 180 degrees. Generally, the voltage phase cutting range of a leading edge dimmer 102 is 10 degrees to 170 degrees. “Phase cutting” the supply voltage refers to modulating a leading edge phase angle of each cycle of an alternating current (“AC”) supply voltage. “Phase cutting” of the supply voltage is also commonly referred to as “chopping”. Phase cutting the supply voltage reduces the average power supplied to a load, such as a lighting system, and thereby controls the energy provided to the load.
The input signal voltage VΦ—IN to the lighting system 100 represents a dimming level that causes the lighting system 100 to adjust power delivered to a lamp 122, and, thus, depending on the dimming level, increase or decrease the brightness of the lamp 122. Many different types of dimmers exist. In general, dimmers use a digital or analog coded dimming signal that indicates a desired dimming level. For example, the triac-based dimmer 102 phase cuts a leading edge of the AC input supply voltage VIN. The leading edge dimmer 102 can be any type of leading edge dimmer, such as a triac-based leading edge dimmer available from Lutron Electronics, Inc. of Coopersberg, Pa. (“Lutron”). A triac-based leading edge dimmer is described in the Background section of U.S. patent application Ser. No. 12/858,164, entitled Dimmer Output Emulation, filed on Aug. 17, 2010, and inventor John L. Melanson.
The phase cut dimmer 102 supplies the input voltage VΦ—IN as modified by the phase cut dimmer 102 to a full bridge diode rectifier 106. The full bridge rectifier 106 supplies an AC rectified voltage VΦR—IN to the switching power converter 108. Capacitor 111 filters high frequency components from rectified voltage VΦR—IN. To control the operation of switching power converter 108, controller 110 generates a control signal CS0 to control conductivity of field effect transistor (FET) switch 112. The control signal CS0 is a pulse width modulated signal. Control signal CS0 waveform 114 represents an exemplary control signal CS0. The controller 110 generates the control signal CS0 with two states as shown in the waveform 114. Each pulse of control signal CS0 turns switch 112 ON (i.e. conducts) represents a first state that causes the switch 112 to operate efficiently and minimize power dissipation by the switch 112. During each pulse of control signal CS0, the inductor current iL increases, as shown in the exemplary inductor current waveform 115, to charge inductor 116 during a charging phase TC. Diode 118 prevents current flow from link capacitor 120 into switch 112. When the pulse of control signals CS0 ends, the control signal CS0 is in a second state, and the inductor 116 reverses voltage polarity (commonly referred to as “flyback”). The inductor current iL decreases during the flyback phase TFB, as shown in inductor current waveform 115. The inductor current iL boosts the link voltage across the link capacitor 120 through diode 118. When the flyback phase TFB ends and when the next charging phase TC begins depends on the operating mode of the switching power converter. In discontinuous conduction mode (DCM), the flyback phase TFB ends before the next charging phase TC begins. However, regardless of whether the switching power converter 108 operates in discontinuous conduction mode, continuous conduction mode, or critical conduction mode, the flyback phase TFB begins as soon as the charging phase TC ends.
The switching power converter 108 is a boost-type converter, and, thus, the link voltage VLINK is greater than the rectified input voltage VΦR—IN. Controller 110 senses the rectified input voltage VΦR—IN at node 124 and senses the link voltage VLINK at node 126. Controller 110 operates the switching power converter 108 to maintain an approximately constant link voltage VLINK for lamp 122, provide power factor correction, and correlate the link current iLINK with the phase cut angle of the rectified input voltage VΦR—IN. Lamp 132 includes one or more light emitting diodes.
To decrease the light output of the lamp 122, the phase cut dimmer 102 increases the phase cut angle of the rectified input voltage VΦR—IN, i.e. time TOFF increases and time TON decreases. The controller 110 responds to the increased phase cut angle by decreasing the current iLNK provided to the lamp 122, which decreases the light output of the lamp 122.
The switching power converter 108 includes a power dissipation resistor 128 so that the dimmer current iDIM does not fall below the hold current value and prematurely disconnect during a cycle of the rectified input voltage VΦR—IN. The “POWER IN” supplied to the switching power converter 108 equals VΦ—IN·iDIM. The “POWER OUT” supplied by switching power converter 108 equals VLINK·iLINK. Because of the relatively low power requirements of an LED based lamp 122, particularly at low light output levels, if the POWER IN equals the POWER OUT+PINH, the dimmer current iDIM may fall below the hold current value and cause the phase-cut dimmer 102 to prematurely disconnect. In this situation, to prevent the dimmer current iDIM from falling below the hold current value, the controller 110 causes the switching power converter 108 to maintain the dimmer current iDIM above the hold current value, which causes the POWER IN to be greater than the POWER OUT+PINH. Since the POWER IN is greater than the POWER OUT+PINH, the switching power converter 108 dissipates the excess power through power dissipation resistor 128.
Because of component non-idealities, the switching power converter 108 includes inherent power losses. Inherent power losses include conductor resistances and switching losses in switch 112. However, circuits are generally designed to minimize inherent power losses, and these inherent power losses are often negligible and, thus, insufficient to dissipate enough power to compensate for the difference between the POWER IN and the POWER OUT+PINH at some POWER OUT levels. To increase the power loss of switching power converter so that the dimmer current iDIM remains above a hold current value even at lower power demand by the lamp 122, switching power converter 108 includes the resistor 128 to create a passive power loss when switch 112 conducts the inductor current iL. For negligible inherent power losses, the resistance value of the resistor 128 is selected so that when the switching power converter is providing a minimum link current iLINK, the POWER IN=POWER OUT+PINH+PASSIVE POWER DISSIPATE.
Resistor 128 is relatively inexpensive to implement as part of switching power converter 108. However, when the link current iLINK is sufficiently high such that POWER IN equals POWER OUT+PINH, the dimmer input current iDIM could be maintained above the hold current value without dissipating power through resistor 128. However, since the dimmer input current iDIM always flows through the resistor 128 when the switch 108 conducts, the resistor 128 still passively dissipates power regardless of whether the POWER IN is equal to the POWER OUT+PINH, which decreases the efficiency of lighting system 100.